1. Field of the Invention
This invention relates generally to implantable drug infusion pumps and more specifically to means on such pumps for measuring the amount of drug remaining in the reservoir of such pumps.
2. Description of the Related Art
Implantable devices such as drug pumps are frequently used to deliver drugs or other liquid medications over long periods of time to selected locations in the human body. These devices commonly include a drug reservoir, catheter means connected to the reservoir to transport the drug and a pumping mechanism to propel the drug in some metered or constant flow dosage to the desired location.
Often the drug reservoir is a bellows shaped reservoir such as is shown in FIGS. 1 and 2. In a bellows reservoir, as the drug leaves the reservoir, the bellows collapses making the volume contained within the reservoir diminish. An example of a drug pump using a bellows-type reservoir as the drug reservoir is the Synchromed.RTM. pump sold by Medtronic, Inc. of Minneapolis, Minn.
In FIGS. 1 and 2, the bellows type reservoir is generally referred to as 2. Reservoir 2 is attached at one end to a bulk head 4. Reservoir 2 has an end plate 6 opposite bulk head 4. A series of pleats 8 connect end plate 6 to bulk head 4 and define a space 10 for containing the drug or other fluid to be contained within the reservoir 2. Bulk head 4 contains an opening 12 through which the drug to be stored within reservoir 2 can either enter or leave space 10. Bulk head 4 may have an additional opening 12A so that drug or other fluid may enter reservoir 2 through opening 12 and leave reservoir 2 through opening 12A.
In an alternate embodiment, shown in FIGS. 3 and 4, a bladder type drug reservoir 2' is attached to a bulk head 4. Bulk head 4 preferably contains openings 12 and 12A through which the drug to be stored within reservoir 2' can either enter or leave space 10'. As drug is added to reservoir 2', a point A on the outer surface of reservoir 2' opposite bulk head 4 moves away from bulk head 4. Conversely, as drug is removed from reservoir 2', point A moves closer to bulk head 4.
It is often desirable for physicians to know exactly how much drug remains in an infusion drug pump reservoir when a patient returns for a refill. To date, electronics on the pump containing the reservoir have estimated the amount of drug in the remaining reservoir based on how much drug should theoretically have been removed from the reservoir by the pumping or metering mechanism of the pump. This amount is then subtracted from the amount of the drug originally inserted into the reservoir to arrive at the amount remaining in the reservoir. The physician then confirms the amount delivered to the patient by removing the old drug from the reservoir by syringe and then subtracting the amount removed from the volume originally injected into the pump. This method has the disadvantage that it is cumbersome, a nuisance and results in the waste of the remaining drug withdrawn from the reservoir. This is a problem in need of a solution.